The following is an excerpt from Tammie L. S. Benzinger et al., Blast-Related Brain Injury: Imaging for Clinical and Research Applications: Report of the 2008 St. Louis Workshop, JOURNAL OF NEUROTRAUMA 26:2127, 2129 (December 2009) (Accessed Aug. 20, 2010, at www.liebertonline.com/doi/pdf/10.1089/neu.2009.0885) (citations omitted):                “It is important to have a basic understanding of the physics of blast insult prior to developing any hypothesis regarding bTBI mechanisms, countermeasures, or treatments. Understanding the processes by which a blast event ultimately inflicts stresses at the cellular and subcellular levels is also a prerequisite to the design of proper animal model testing and interpretation of results.        A blast event as considered here begins with a detonation, the nearly instantaneous combustion of a liquid or solid explosive material resulting in the generation of gaseous products at extremely high pressure and temperature (˜150 k atm/˜2M psi, ˜3000° K). The gaseous detonation products expand rapidly into the surrounding atmosphere to about 3000-fold their original volume, and are visible as a luminous fireball. Primary fragmentation from the charge casing as well as dirt and ejecta from buried charges will be carried with the fireball expansion and are projected much further than the gaseous products. The rapid expansion of the fireball drives a shockwave into the surrounding air ahead of it. The combined violent expansion of product gases and propagated shockwave constitute the blast flow field.        The most distinctive feature of the air blast wave energy is the shock front, through which there is a nearly instantaneous change in all gas-dynamic conditions of the air (pressure, density, flow velocity, and temperature). While the air blast wave energy strength is often characterized exclusively in terms of the peak blast overpressure, it is important to note that this metric will usually refer to the static or side-on pressure above ambient levels, which does not represent the loading condition on a typical target. The static pressure is that pressure which would be sensed by a surface aligned parallel to the blastwave propagation, and hence does not experience the kinetic energy component of the flow, which may be many-fold higher than the static pressure component. If the same surface were perpendicular to the blast, it would obstruct the flow and be exposed to a much higher pressure of the reflected blast, including both the static and dynamic (kinetic energy) components. The actual stresses and waveform experienced at the cellular level will depend on the transfer function for the target, which is highly geometry- and material-dependent. These distinctions regarding the incident blast flow conditions, imparted loading, and cellular stresses have important implications with regard to the mechanisms for blast injury, as well as the proper simulation of blast in the laboratory, Whereas the static pressure profile is an important component of blast insult, it is by no means the only relevant energy component, particularly for victims within the area of the fireball, where kinetic energy of the flow is dominant.        The blast flow field exhibits energy in various modes in the hydrodynamic domain, including material flow (kinetic energy), static pressure, and internal energy (temperature). Due to the shock front, the frequency content of the incident wave is extremely high; indeed, the rate of the stress rise imparted to tissue followed by rapid relaxation may be of as much concern with regard to cellular damage as stress amplitude. Blast also can propagate energy in the electromagnetic domain, although the power spectrum is highly dependent on the device size and configuration.”        
FIG. 1 includes a graph 10 illustrating pressure versus time of an example air blast wave energy 195. The example blast wave represents an air blast wave energy produced by a blast event, such as a detonating high-order explosive. The graph represents time on a horizontal axis, and static or side-on pressure on a vertical axis expressed in units of overpressure PSO, or atmospheres above or below ambient pressure. The air blast wave includes a shock front 22, which is typically traveling at a supersonic speed, is nearly vertical in its onset, and has a thickness generally estimated at less than one micron. The shock front is the leading edge of the air blast wave; it is the portion of the air blast wave transitioning from ambient atmospheric pressure to maximum overpressure. The air blast wave includes a region of overpressure 24, and a region of underpressure 26. Humans exposed to air blast waves generated by detonating high-order explosives are at risk for blast-related traumatic brain injury (bTBI), which is particularly relevant in current military engagements around the world, and which some consider the signature injury of the wars in Iraq and Afghanistan.
Air blast waves, like light, ultrasonic, and sonic waves, are reflected at boundaries where there is a difference in acoustic impedances (Z) of the materials on each side of the boundary. This difference in Z is commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one medium and another. Acoustic impedance (Z) values are generally expressed or used herein in MRayls unless otherwise indicated.
The fraction of the incident wave intensity that is reflected can be derived because particle velocity and local particle pressures must be continuous across the boundary. When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected can be calculated with the equation below. The value produced is known as the reflection coefficient (R). Multiplying the reflection coefficient by 100% yields the calculated amount of energy reflected as a percentage of the original energy.R=[(Z2−Z1)/(Z2+Z1)]2 
Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one.